To appreciate the advantages of a multi-truss retaining ring having the capability of deflecting to a greater degree than conventional tapered section-height retaining rings without taking on an excessive permanent set and in such manner as to increase (external form) or decrease (internal form) the circumferential length of the ring body, it may be helpful to sketch briefly the development of the retaining ring art leading to the present invention, as follows:
The oldest retaining rings still in use to retain machine parts on shafts, etc., or in housing bores are rings of uniform radial-section height made of coils of wire. Under stress, such uniform section-height wire rings do not deform uniformly circularly but instead they deform elliptically, and hence under stress they do not seat uniformly against the bottoms of the grooves in which they are conventionally assembled. Also, these uniform section-height rings made of coils of wire are severely limited as to their section heights (which are relatively small) and to use in grooves of shallow depth.
The first documented effort directed to overcoming the aforesaid disadvantages of uniform section-height rings is represented by Heiermann U.S. Pat. No. 1,758,515 (reissued as U.S. Pat. No. Re. 18,144) disclosing and claiming a tapered retaining ring, i.e. a ring having section heights which diminish progressively from a relatively high middle section towards its open ends. While the tapered-section ring of Heiermann, by its capability of deforming substantially circularly under stress, widened the fields of safe application of retaining rings to a considerable degree by providing a ring body having greater section-height than possible with uniform section-height rings and thereby a more effective parts-retaining shoulder without becoming loose in its groove in consequence of a permanent set, provided of course that during assembly and disassembly the elastic limit of the ring body material was not exceeded, it was soon learned that even the tapered retaining rings as per Heiermann U.S. Pat. No. Re. 18,144 could be assembled in relatively shallow grooves only, resulting in proportionately limited thrust load capacities of the ring-in-groove assemblies employing same.
The above-mentioned limitations of the Heiermann ring resulted in the art developing further to the extent as evidenced by the Wurzel patents disclosing and claiming so-called "pre-stressed" retaining rings, i.e. U.S. Pat. No. 2,861,824 for the internal form rings, and U.S. Pat. No. 2,982,165 for the external form rings. These "pre-stressed" retaining rings were characterized by increased section heights and by diameters requiring a moderately increased (as compared to the Heiermann rings) expansion or contraction in the course of their first assembly, consequent to which they took on a predetermined slight permanent set. Such ring design made possible the installation of the pre-stressed ring in deeper shaft or housing bore grooves, in turn resulting in substantial increases of thrust load capacity of the ring-in-groove assemblies as compared to the ring-in-groove assemblies employing Heiermann rings.
As far as was then known, the aforesaid deeper groove was the maximum depth of groove in which any and all retaining rings could be assembled. That is to say, while the want therefor may have been present, a prestressed ring designed to be installed in a groove of greater depth than said maximum was considered to be impractical if not impossible of attainment because the bending characteristics of the pre-stressed rings were too limited. Nor could the want for a ring designed to provide the substantially higher shoulder be satisfied, because conventional designs of pre-stressed rings would be too stiff to handle if their section heights were increased to the degree satisfying the higher-shoulder-want. Further and equally important, the higher shoulders would require such higher ring sections as would invariably result in permanent set and looseness of rings in even the aforesaid "deeper groove" achieved under Wurzel, U.S. Pat. No. 2,861,824.
To the foregoing brief recital of the development of the retaining ring art as documented in the patents aforesaid must be added the knowledge that retaining rings generally, when expanded or compressed (contracted), are subject to circumferential stresses which, occuring in the form of tensile and/or compressive stresses acting on the circumferential ring fibers at any section of the ring, result from the forces applied to expand or compress such rings, which forces result in a bending moment acting on any section of the ring. It is these bending moments which cause a change in the curvature of such retaining rings as results in their expansion of contraction (compression). While the circumferential stresses resulting from the bending moments acting on sections around the ring body will in theory cause an elastic lengthening or shortening of the fibers of the ring material, if occurring such as a practical matter is too small to be measured and hence no consideration or study was given to the utilization thereof, nor was any purposeful structuring of the ring body designed to increase or decrease the circumferential length thereof to a significant degree heretofore initiated by such knowledge.